Just take them out and either leave them overnight in a hot press or near a radiator, or just simply use a heat-gun or hairdryer to heat the glue and then run a stanley knife around the edge to break the seal and then gently prise the lense off. I only painted the removable surround, which is dead handy to remove. I simply rubbed them down with a scourer and then gave them 2 coats of etch primer and then 3 coats of Elite Satin Black (best black paint imo) and then used windscreen bond to stick them again. 2-3hrs tops to complete it and just be sure to have the heat going when your priming and painting them and they'll be dry in no time at all.
I think it helps the front a lot makes them a lot more menacing aswell...